The formation and development of images on photoconductive materials by electrostatic means is well known. The best known of the commercial process utilizes a latent electrostatic image on an imaging surface by first uniformly electrostatically charging the surface in the dark and then exposing the electrostatically charged surface to a light and shadow image. The light-struck areas of the imaging layer are thus made conductive and the electrostatic charge selectively dissipated in these areas. The latent positive electrostatic image remaining is made visible by development with a finely divided colored electroscopic material known as "toner". This material is preferentially attracted to those areas on the image-bearing surface which have retained an electrostatic charge. After development, the image is permanently affixed to the photoconductor or transferred to some other suitable material such as paper.
Photoconductor layers useful for xerographic purposes (1) may be homogeneous layers of a single material such as vitreous selenium or (2) may be composite layers containing a photoconductor and another material. One type of composite layer used in xerography is illustrated by U.S. Pat. No. 3,121,006 to Middleton and Reynolds, which described a number of binder layers containing finely-divided particles of a photoconductive inorganic compound such as zinc oxide, dispersed in an electrically insulating organic resin binder. In the systems described in Middleton et al, the binder comprises a material which is incapable of transporting the injected charge carriers generated by the photoconductor particles for any significant distance. As a result, the photoconductor particles must be in substantially continuous particle-to-particle contact throughout the layer to permit sufficient charge dissipation is the light-struck areas. The uniform dispersion of photoconductor particles described in Middleton et al, therefore, represents a high volume concentration (i.e. up to about 50 percent or more by volume) of photoconductive particles.
It has also been found, however, that high photoconductor loadings in a binder layer can adversely affect physical continuity and significantly reduce the mechanical properties of a binder layer. High photoconductor loadings, therefore, are often characterized by brittleness and lack of durability. On the other hand, when the photoconductor concentration is substantially reduced below about 50 percent by volume, the surface discharge rate is correspondingly reduced, making high speed cyclic or repeated imaging difficult or impossible.
In the second Middleton et al patent (U.S. Pat. No. 3,121,007) another type of photoconductor is considered, which includes a two phase photoconductive binder layer comprising photoconductive insulating matrix. The photoconductor is in the form a particulate photoconductive inorganic crystalline pigment broadly disclosed as being present in an amount from about 5 to 80 percent by weight. Here photo discharge is probably effected in a combination of charge carriers generated in the photoconductive insulating matrix material and charge carriers injected directly from the photoconductive crystalline pigment into the photoconductive insulating matrix.
U.S. Pat. No. 3,037,861 to Hoegl et al indicates that polyvinyl carbazole exhibits some long-wave U.V. sensitivity and suggests that spectral sensitivity can be extended into the visible light spectrum by the addition of dye sensitizers. This patent further suggests that other additives such as zinc oxide or titanium dioxide can be used in conjunction with polyvinyl carbazole as a photoconductor (with or without additive materials) to extend spectral sensitivity.
In addition to the above, certain specialized layered structures have been proposed for reflex imaging. In U.S. Pat. No. 3,165,405 to Hoesterey, for instance, there is a two layered zinc oxide binder structure. Hoesterey requires two separate contiguous photoconductive layers having different spectral sensitivities in order to carry out a particular reflex imaging sequence. This device utilizes the properties of multiple photoconductive layers in order to obtain the combined advantages of the separate photoresponse of the respective photoconductive layers.
Although the above patents rely upon distinct mechanisms of discharge throughout the photoconductive layer, they suffer from a common deficiency insofar as the photoconductive surface is very susceptible to abrasion, chemical attack, heat, and multiple exposures to light during cycling. As a result it is common to experience a gradual deterioration in the electrical characteristics of the photoconductive layer. This manifest, for instance, in printing of surface defects and scratches, and in the existence of localized areas of persistent conductivity.
Another form of composite photosensitive layer which has also been considered by the prior art includes a layer of photoconductive material which is covered with a relatively thick plastic layer and coated on a supporting substrate.
U.S. Pat. No. 3,041,166 to Bardeen describes such a configuration in which a transparent plastic material overlays a layer of vitreous selenium contained on a supporting substrate. The plastic material is described as one having a long range for charge carriers of the desired polarity. In operation, the free surface of the transparent plastic is electrostatically charged to a given polarity. The device is then exposed to activating radiation which generates a hole-electron pair in the photoconductive layer. The electron moves through the plastic layer and neutralizes a positive charge on the free surface of the plastic layer thereby creating an electrostatic image. Bardeen, however, does not teach any specific plastic materials which will function in this manner, and confines his examples to structures which use a photoconductor material for the top layer.
While the later patent represents a significant breakthrough, it has been found that it is very difficult, if not impossible to obtain and effectively utilize certain types of potentially suitable compounds as charge transmitting materials. This is true of various polycyclic aromatic subgroups, and particularly polycyclic aromatics, such as 1,2-benzanthracene and its derivatives which exhibit substantial .pi. electron delocalization.
It is an object of the present invention to synthesize and utilize active polycyclic aromatic matrix components, particular vinyl polycyclic aromatic derivatives, suitable for transporting photoconductor-generated holes or electrons for general electrophotographic and xerographic purposes.
It is a further object to obtain new polymeric derivatives for use as active matrix components for xerographic purposes.